Comments

I use +15. It's easy and reasonable in price to acquire, and it's fast enough for my purposes (i.e., I can escape mobs and get out of 'combat' if i so desire). IMO anything more isn't needed for any current content, so I sell my infusions. Even getting the +15 is of questionable value.

Swim+15 is my recommendation, too, for those who want to spend any money at all on swimming faster in the 1% of play time they spend underwater.
(Even more economical: don't spend anything at all on underwater breathers, let alone swim speed infusions.)

Swim+10 boosts speed 10% and likewise Swim+30 boosts it 30%. However, your max speed is capped at +40% and we get 33% from swiftness. Thus, in most situations, Swim+10 will hit the cap.

Further, the costs go up exponentially, while the benefits are incremental. Each extra 1% boost is roughly double the value. In effect, for the same price, you can have 5 toons with Swim+15 or one toon with Swim+20 (although, of course, you can use an extractor to move it around, at 24s per change).

I guess I'm the outlier then: I'm sticking with no boost. That saves 50-70 gold on the breather (or 15-25g worth of time spent on berries & unbound magic), and at least 10g for a Swim+15 infusion. I like underwater combat, but not 60-80 gold worth per character.

added: as noted by @Tanner Blackfeather.6509, there's a coin-free ascended breather. For me, that's an even bigger resource sink, since I have many sources of gold, but only one for berries & UM. Much as I like underwater, I can't justify the cost of ascended breathers, when they aren't important for combat.

I guess I'm the outlier then: I'm sticking with no boost. That saves 50-70 gold on the breather, and at least 10g for a Swim+15 infusion. I like underwater combat, but not 60-80 gold worth per character.

My plan is "get breathers via Winterberries when and as I get around to it" followed by slotting a single +10 in each until all alts have. Then maybe I'll look at increasing my main...